Create classes_util API, change discover to return a list instead of a dict.

tools/perf/benchmarks/skpicture_printer.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from core import perf_benchmark
 
 from telemetry import benchmark
-from telemetry.core import discover
 from telemetry import story
+from telemetry.util import classes_util
 
 from measurements import skpicture_printer
 
 
 def _MatchPageSetName(story_set_name, story_set_base_dir):
-  story_sets = discover.DiscoverClasses(story_set_base_dir, story_set_base_dir,
-                                        story.StorySet).values()
+  story_sets = classes_util.DiscoverClasses(
+      story_set_base_dir, story_set_base_dir, story.StorySet)
   for s in story_sets:
     if story_set_name == s.Name():
       return s
   return None
 
 
 @benchmark.Disabled
 class SkpicturePrinter(perf_benchmark.PerfBenchmark):
   @classmethod
   def AddBenchmarkCommandLineArgs(cls, parser):
@@ skipping 14 matching lines @@
   def Name(cls):
     return 'skpicture_printer'
 
   def CreatePageTest(self, options):
     return skpicture_printer.SkpicturePrinter(options.skp_outdir)
 
   def CreateStorySet(self, options):
     story_set_class = _MatchPageSetName(options.page_set_name,
                                         options.page_set_base_dir)
     return story_set_class()